An airborne collision avoidance system (ACAS, usually pronounced as ''ay-kas'') operates independently of ground-based equipment and

air traffic control Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airsp ...

in warning pilots of the presence of other aircraft that may present a threat of collision. If the risk of collision is imminent, the system initiates a maneuver that will reduce the risk of collision. ACAS standards and recommended practices are mainly defined in annex 10, volume IV, of the

Convention on International Civil Aviation The Convention on International Civil Aviation, also known as the Chicago Convention, established the International Civil Aviation Organization (ICAO), a specialized agency of the United Nations charged with coordinating international air trave ...

. Much of the technology being applied to both military and general aviation today has been undergoing development by NASA and other partners since the 1980s. A distinction is increasingly being made between ACAS and ASAS (airborne separation assurance system). ACAS is being used to describe short-range systems intended to prevent actual metal-on-metal collisions. In contrast, ASAS is being used to describe longer-range systems used to maintain standard ''en route'' separation between aircraft ( horizontal and vertical). oekstra, J.M. (2002). Free flight with airborne separation assurance. Report No. NLR-TP-2002-170. National Aerospace Laboratory NLR./ref>

Implementations

As of 2022, the only implementations that meets the ACAS II standards set by

ICAO The International Civil Aviation Organization (ICAO, ) is a specialized agency of the United Nations that coordinates the principles and techniques of international air navigation, and fosters the planning and development of international a ...

are Versions 7.0 and 7.1 of TCAS II ( Traffic Collision Avoidance System) produced by Garmin,

Rockwell Collins Rockwell Collins was a multinational corporation headquartered in Cedar Rapids, Iowa, providing avionics and information technology systems and services to government agencies and aircraft manufacturers. It was formed when the Collins Radio Co ...

Honeywell Honeywell International Inc. is an American publicly traded, multinational conglomerate corporation headquartered in Charlotte, North Carolina. It primarily operates in four areas of business: aerospace, building technologies, performance ma ...

and ACSS (Aviation Communication & Surveillance Systems; an L-3 Communications and Thales Avionics company). As of 1973, the United States

Federal Aviation Administration The Federal Aviation Administration (FAA) is the largest transportation agency of the U.S. government and regulates all aspects of civil aviation in the country as well as over surrounding international waters. Its powers include air traffic ...

(FAA) standard for transponder minimal operational performance, Technical Standard Order (TSO) C74c, contained errors which caused compatibility problems with

air traffic control radar beacon system The air traffic control radar beacon system (ATCRBS) is a system used in air traffic control (ATC) to enhance surveillance radar monitoring and separation of air traffic. It consists of a rotating ground antenna and transponders in aircraft. The ...

(ATCRBS) radar and Traffic Collision Avoidance System (TCAS) abilities to detect aircraft transponders. First called "The Terra Problem", there have since been individual FAA Airworthiness Directives issued against various transponder manufacturers in an attempt to repair the operational deficiencies, to enable newer radars and TCAS systems to operate. Unfortunately, the defect is in the TSO, and the individual corrective actions to transponders have led to significant differences in the logical behavior of transponders by make and mark, as proven by an FAA study of in-situ transponders. In 2009, a new version, TSO C74d was defined with tighter technical requirements. AIS-P (ACAS) is a modification which both corrects the transponder deficiencies (the transponder will respond to all varieties of radar and TCAS), then adds an Automatic Independent Surveillance with Privacy augmentation. The AIS-P protocol does not suffer from the saturation issue in high density traffic, does not interfere with the Air Traffic Control (ATC) radar system or TCAS, and conforms to the internationally approved Mode S data packet standard. It awaits member country submission to the ICAO as a requested approval.

Other collision avoidance systems

Modern aircraft can use several types of collision avoidance systems to prevent unintentional contact with other aircraft, obstacles, or the ground.

Aircraft collision avoidance

Some of the systems are designed to avoid collisions with other aircraft and

UAV An unmanned aerial vehicle (UAV), commonly known as a drone, is an aircraft without any human pilot, crew, or passengers on board. UAVs are a component of an unmanned aircraft system (UAS), which includes adding a ground-based controller ...

s. They are referred to as "electronic conspicuity" by the UK CAA. * Airborne

radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor v ...

can detect the relative location of other aircraft, and has been in military use since

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...

, when it was introduced to help night fighters (such as the

de Havilland Mosquito The de Havilland DH.98 Mosquito is a British twin-engined, shoulder-winged, multirole combat aircraft, introduced during the World War II, Second World War. Unusual in that its frame was constructed mostly of wood, it was nicknamed the "Wooden ...

and Messerschmitt Bf 110) locate bombers. While larger civil aircraft carry

weather radar Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern weather radars are mostly pulse- ...

, sensitive anti-collision radar is rare in non-military aircraft. * Traffic collision avoidance system (TCAS), the implementation of ACAS, actively interrogates the

transponder In telecommunications, a transponder is a device that, upon receiving a signal, emits a different signal in response. The term is a blend of ''transmitter'' and ''responder''. In air navigation or radio frequency identification, a flight trans ...

s of other aircraft and negotiates collision-avoidance tactics with them in case of a threat. TCAS systems are relatively expensive, and tend to appear only on larger aircraft. They are effective in avoiding collisions only with other aircraft that are equipped with functioning transponders with altitude reporting. * PCAS Xrx

a Portable Collision Avoidance System (PCAS) is a less expensive, passive version of TCAS designed for

general aviation General aviation (GA) is defined by the International Civil Aviation Organization (ICAO) as all civil aviation aircraft operations with the exception of commercial air transport or aerial work, which is defined as specialized aviation services ...

use. PCAS systems do not actively interrogate the transponders of other aircraft, but listen passively to responses from other interrogations. PCAS is subject to the same limitations as TCAS, although the cost for PCAS is significantly less. * FLARM is a small-size, low-power device (commonly used in gliders or other light aircraft) which broadcasts its own position and speed vector (as obtained with an integrated GPS) over a license-free ISM band radio transmission. At the same time it listens to other devices based on the same standard. Intelligent motion prediction algorithms predict short-term conflicts and warn the pilot accordingly by acoustical and visual means. FLARM incorporates a high-precision WAAS 16-channel GPS receiver and an integrated low-power radio transceiver. Static obstacles are included in FLARM's database. No warning is given if an aircraft does not contain an additional FLARM device.

Terrain collision avoidance

* a Ground proximity warning system (GPWS), or ''Ground collision warning system'' (GCWS), which uses a

radar altimeter A radar altimeter (RA), also called a radio altimeter (RALT), electronic altimeter, reflection altimeter, or low-range radio altimeter (LRRA), measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it t ...

to detect proximity to the ground or unusual descent rates. GPWS is common on civil airliners and larger

aircraft. * a Terrain awareness and warning system (TAWS) uses a digital terrain map, together with position information from a navigation system such as GPS, to predict whether the aircraft's current flight path could put it in conflict with obstacles such as mountains or high towers, that would not be detected by GPWS (which uses the ground elevation directly beneath the aircraft). One of the best examples of this type of technology is the Auto-GCAS ( Automatic Ground Collision Avoidance System) and PARS (Pilot Activated Recovery System) that was installed on the entire USAF fleet of F-16's in 2014. *

Synthetic vision A synthetic vision system (SVS) is a computer-mediated reality system for aerial vehicles, that uses 3D to provide pilots with clear and intuitive means of understanding their flying environment. Functionality Synthetic vision provides situat ...

provides pilots with a computer-generated simulation of their outside environment for use in low or zero-visibility situations. Information used to present warnings is often taken from GPS, INS, or gyroscopic sensors.

References

External links